Extrusion apparatus for the continuous formation or molding of coatings about elongated core members, such as polymeric insulating enclosures or covers on electrical wire or cables, typically comprise one of two types or systems, namely, a cross or transverse-head type of extruder such as shown in U.S. Pat. Nos. 2,185,701; 2,740,988; and 3,581,343; or an in-line type of extruder such as shown in the second stage extruding unit of U.S. Pat. No. 3,979,488 and FIG. 1 of U.S. Pat. No. 3,981,653.
The cross or transverse-head type of extruder, which is most commonly used for insulation coating or covering of wire and cable, consists of an apparatus having a head member or assembly incorporating the mold forming unit or die arranged generally transverse or at a substantial angle such as about 45.degree. to 90.degree., with respect to the direction or axis of the screw of the main extruder, or of the last extruder unit of a series of a multiple extruder apparatus, and is thus adjoined in an operative relationship with the downstream end or outlet from the screw. Accordingly, the elongated core member or wire to be coated by the extruder apparatus, travels only through the transversely arranged head member or assembly containing the forming unit or die and by-passes the extruder screw.
In the in-line type of extrusion apparatus, the screw of the main extruder or of the last extruder unit of a series of a multiple extruder apparatus, is arranged in substantial axial alignment with the mold forming unit or die, and is provided with a longitudinal bore or passage extending coaxially therethrough and generally coaxially aligned with the orifice of the mold forming unit or die. Thus, the elongated core member or wire to be coated passes through the length of the main extruder screw via the coaxial bore or passage extending therethrough, and continues in the same general direction on through the similarly aligned or substantially coaxial mold forming unit or die for the application of the coating thereabout.
With an in-line type of extrusion apparatus it is particularly difficult, if not substantially impossible, to achieve and/or maintain the continuous molding or forming of a coating or covering body around an elongated core member or wire which is constantly of uniform annular thickness or substantially concentric with the core or wire, because the core member or wire cannot effectively be held precisely central to the die orifice and the inevitable presence or occurrence of disturbing motions or forces. Misaligning conditions for the core member or wire in the in-line type of extruder are primarily attributable to the wobbling or wandering movement of the free downstream end of the rotating extruder screw which is inherent even when the apparatus is maintained at normal tolerances. The wobbling or wandering motion of the screw terminal is imparted to the core member or wire moving therethrough and emerging from the screw's free end just prior to its entering into and moving through the stationary orifice of the mold forming unit or die for the molding formation of the surrounding coating thereabout, whereby concentricity cannot be achieved for significant periods or maintained.
Due to this shortcoming of the in-line type of extruder apparatus for coating wire or the like, the cross or transverse-head type of extruding apparatus is in general more commonly used for coating elongated core members such as forming insulating coverings on electrical wire wherein uniformity of thickness or concentricity is a particularly significant factor. In the usual cross or transverse-head type of extruding apparatus the core member or wire does not encounter the unstable or free end wobbling or wandering movement of the rotating screw, and in traveling generally transverse to the screw while advancing, from a guide into and through the orifice of a mold forming unit or die within the head, the adverse effects of the screw's motion are avoided or bypassed.
However, the cross or transverse-head type of extruder apparatus which is most commonly used for coating wire, also is subject to a potentially serious shortcoming which results in the same impediment as the in-line extruder, namely, the inability to effectively maintain continuous concentricity or uniformity of coating thickness of the extrusion molded covering on the core member or wire. In this type of cross or transverse-head extruder, the problem is attributable to the non-symmetrical or uneven flow passages or courses required to advance the plastic material or molding stock from the transversely disposed extruder screw laterally through a side entry to the transverse-head and therefrom into an annular area surrounding the core member or wire and mold forming unit or die. Such non-symmetrical or uneven flow passages or courses for the supplying of plastic material or molding stock to the transversely arranged mold forming unit or die, and the resulting inevitable differences in flow resistance encountered therein, creates a propensity for uneven or unbalanced rates of flow or movement of the advancing plastic material or molding stock within and through the conduits of the system. Uneven or unbalanced flow or movement of material or stock in turn causes pressure differences or irregularities in and about the mold forming unit or die whereby the advancing core member or wire can be deflected or forced out of alignment with the orifice of the forming unit or die with the result of an uneven coating being formed thereabout which lacks concentricity. For example, note U.S. Pat. Nos. 2,740,988 and 3,608,136, and the complex remedies set forth therein in efforts designed to overcome this shortcoming of cross or transverse-head type extruding apparatus.
Moreover, the non-symmetrical or uneven flow passages or courses of the cross or transverse-head type of extruder apparatus can be the source of other problems or difficulties. For instance, when extrusion molding with stocks of heat curable polymeric molding compounds, the non-symmetrical or uneven flow passages or courses of this type of apparatus produce variant or inconsistent residency periods and temperature conditions within the mass of heat sensitive molding stock as it advances through the apparatus, causing premature or irregular curing therethrough. Also, molding stock is more prone to become static or retained within this type of apparatus for indeterminate periods of time whereby it can reach such an advanced stage of heat curing that it is no longer sufficiently plastic to be miscible with the balance of the more plastic and yet uncured stock. An advanced stage of cure in portions of the molding stock and a lack of uniform miscibility of the stock disrupts the uniform or smooth continuous flow thereof through the die or mold forming device, and thereby causes imperfections and/or discontinuities within the product molded therefrom as well as a lack of physical homogeneity therein.